James Thomson wrote the words for "Rule Britannia" in 1740; this monument was erected in his memory (Thomson was born in the nearby Ednam.)
Thomson was a poet in general. The information board here (with great views of the Eildon Hills from the same place, by the way) also informs that his poem "Liberty" was dedicated to the Prince of Wales who rewarded him with a pension and the sinecure of Surveyor-General of the Leeward Islands. Interesting stuff!